1. Field of Use
This invention relates generally to overhead travelling cranes. In particular, it relates to restraint means connected between the crane and its supporting structure to dampen relative shifting movement of the crane and thereby prevent crane derailment as the structure oscillates from earthquake shocks or the like.
2. Description of the Prior Art
Some overhead travelling cranes comprise an elongated bridge, a trolley-mounted hoist movable along the bridge, and main wheels (some power driven) mounted at opposite ends of the bridge and engaged with runway rails which are mounted on a supporting structure. in some cases, the runway rails are parallel and the crane moves in a straight path therealong. In other cases, as in some atomic power plants, a single circular runway rail is provided and the crane either rotates about or moves around a central point, depending on the spacing between the peripheral locations where the main wheels engage the rail. Usually, the main wheels are single or double-flanged to ensure engagement with the associated runway rail, although some wheels are unflanged and auxiliary side wheels or rollers are employed to keep the main wheels aligned with the runway rail.
In any case, if the structure on which the crane is mounted is subjected to horizontal oscillation resulting from heavy shocks caused by earthquakes, explosions, or other events, there is relative shifting movement between the structure and the crane which can result in crane derailment and damage to the main wheels and runway rails, even if flanged wheels or auxiliary side wheels or rollers are employed, thereby rendering the crane inoperative. In atomic power plants, the danger is aggravated if derailment or damage occurs while radioactive materials or components are being transported. The relative horizontal shifting between the crane and its supporting structure can occur because the force of the shock is great, the mass of the crane is quite large, and the crane wheels merely rest on the runway rails. Typically, the main wheel flanges or auxiliary side wheels or rollers are not strong enough to withstand breakage as they are thrust against the side of a runway rail. Furthermore, since the forces are large, the use of stronger flanges would then result in damage to the runway rails. For example, in a typical atomic power plant wherein the circular runway rail has a diameter of about 134 feet (and a corresponding bridge length and size), the structure could oscillate at a frequency of about 3 to 5 Hz (in simple harmonic motion) and could deflect horizontally as much as .+-.0.53 inches relative to the crane at a velocity of up to 30.6 inches per second at the 5 Hz frequency during an earthquake. A crane of such size and corresponding weight would exert total horizontal forces on the order of 423,000 pounds, for example, which would be distributed between the main wheel flanges and the runway rail. In view of the large relative movements and forces acting between the crane and the structure on which it is mounted during shocks from earthquakes or other causes, and the possible dangers therefrom, it is desirable to provide restraint means to dampen or control horizontal shifting movement of the crane relative to the structure and thereby prevent derailment and wheel and runway damage.
The following patents exemplify some prior art apparatus intended to maintain alignment between the main wheels of overhead travelling cranes and the runway rails therefor under various circumstances: U.S. Pat. No. 3,095,829; U.S. Pat. No. 1,758,580; West German Pat. No. 1,531,277; and U.S.S.R. Pat. No. 3 96 299. The prior art shows that one or two flanges have been used on a main wheel to maintain registration with a runway rail. The prior art also teaches the use of auxiliary wheels or rollers which ride on the side of a runway rail or separate guide rail to maintain wheel alignment. However, the prior art does not teach the concept of dampening or controlling shifting movement of the crane relative to the structure or overhead travelling cranes able to withstand derailment or damage during heavy earthquake shocks.